Charge-ordered State Research Articles

Effect of substrate and orientation on charge ordering behaviors in epitaxial Pr0.5Ca0.35Sr0.15MnO3 films

The charge ordering (CO) behaviors of Pr0.5Ca0.35Sr0.15MnO3 films grown on STO(100), STO(110) and LAO(100) are systematically investigated by transport measurements and transmission electron microscopy (TEM) examinations. From the transport measurements, the CO transition temperatures of all the three films are much higher than those of the bulk materials, showing that the film strain could enhance the CO transition. From TEM observations, many superlattice spots appear in the electron diffraction patterns taken from the films, indicating the appearance of the CO modulation structures at room temperature. The modulation vectors are determined to be (1/2, 0, 0) for STO (100), (1/2, 1/2, 1/2) for STO (110), and both (0, 1/2, 0) and (1/2, 1/2, 0) for LAO (100). It is shown that both the substrate orientation and the film strain have a great effect on the CO modulation structures. The CO state is much easier to appear in the compressive strain direction which is due to the Mn-O-Mn angle tilting.

The effect of Co doping on the magnetic and magnetocaloric properties of Pr0.7Ca0.3Mn1−xCoxO3 manganites

In this work, we report the effect of low amount of cobalt doping at the Mn-site on the magnetic and magnetocaloric properties of Pr0.7Ca0.3Mn1−xCoxO3 (0≤x≤0.1) powder samples. Our samples, elaborated using the solid–solid reaction method at high temperature, are single phase and crystallize in the orthorhombic system with Pnma space group. While the parent compound Pr0.7Ca0.3MnO3 exhibits a charge order state at low temperature, the substituted samples with low amount of cobalt exhibit a paramagnetic to ferromagnetic transition with decreasing temperature. The Curie temperature TC increases with Co content from 105K for x=0 to 116K for x=0.1. The maximum values of the magnetic entropy change |ΔSMmax| are found to be 0.8J/kgK, 2.2J/kgK, 3.1J/kgK and 3.2J/kgK in a magnetic field change of 5T for x=0, 0.02, 0.05 and 0.1 respectively. The maximum value of the relative cooling power RCP is found to be 378.2J/kg in the Pr0.7Ca0.3Mn0.95Co0.05O3 at 5T. This value of RCP is about 92% of that obtained in gadolinium metal, known as one of the most important materials for magnetic refrigeration, at the same magnetic field change of 5T.

Thickness-dependent charge ordering state in Bi0.4Sr0.28Ca0.32MnO3 thin films under the anisotropic strain

We have achieved clear charge ordering (CO) state in Ca doped Bi0.4Sr0.6−xCaxMnO3 (BSCMO) films on (110)-orientated SrTiO3. The BSCMO film's CO transition temperature (TCO) was tuned to room temperature (RT) by optimum doping Ca (x = 0.32). Asymmetric x-ray reciprocal space mapping was used to determine the structure evolution of the films with thickness, in detail, under the anisotropic stress in films. We found that the CO phase only appears in thicker films, accompanied by an unique distortion in structure, in which twin-domains structure occurs with the [110] axis slightly tilting toward [1–10] and [−110] directions. The results demonstrate the effects of anisotropic stress on the structure and electronic phases in BSCMO films as thickness increases. Our works are valuable for allowing a RT study in the CO film and further operation of the devices for a practical use.

Visualization of a ferromagnetic metallic edge state in manganite strips.

Recently, broken symmetry effect induced edge states in two-dimensional electronic systems have attracted great attention. However, whether edge states may exist in strongly correlated oxides is not yet known. In this work, using perovskite manganites as prototype systems, we demonstrate that edge states do exist in strongly correlated oxides. Distinct appearance of ferromagnetic metallic phase is observed along the edge of manganite strips by magnetic force microscopy. The edge states have strong influence on the transport properties of the strips, leading to higher metal-insulator transition temperatures and lower resistivity in narrower strips. Model calculations show that the edge states are associated with the broken symmetry effect of the antiferromagnetic charge-ordered states in manganites. Besides providing a new understanding of the broken symmetry effect in complex oxides, our discoveries indicate that novel edge state physics may exist in strongly correlated oxides beyond the current two-dimensional electronic systems.

Gate-tunable phase transitions in thin flakes of 1T-TaS2.

The ability to tune material properties using gating by electric fields is at the heart of modern electronic technology. It is also a driving force behind recent advances in two-dimensional systems, such as the observation of gate electric-field-induced superconductivity and metal-insulator transitions. Here, we describe an ionic field-effect transistor (termed an iFET), in which gate-controlled Li ion intercalation modulates the material properties of layered crystals of 1T-TaS2. The strong charge doping induced by the tunable ion intercalation alters the energetics of various charge-ordered states in 1T-TaS2 and produces a series of phase transitions in thin-flake samples with reduced dimensionality. We find that the charge-density wave states in 1T-TaS2 collapse in the two-dimensional limit at critical thicknesses. Meanwhile, at low temperatures, the ionic gating induces multiple phase transitions from Mott-insulator to metal in 1T-TaS2 thin flakes, with five orders of magnitude modulation in resistance, and superconductivity emerges in a textured charge-density wave state induced by ionic gating. Our method of gate-controlled intercalation opens up possibilities in searching for novel states of matter in the extreme charge-carrier-concentration limit.

Phase-change memory function of correlated electrons in organic conductors

Phase-change memory (PCM), a promising candidate for next-generation non-volatile memories, exploits quenched glassy and thermodynamically stable crystalline states as reversibly switchable state variables. We demonstrate PCM functions emerging from a charge-configuration degree of freedom in strongly correlated electron systems. Non-volatile reversible switching between a high-resistivity charge-crystalline (or charge-ordered) state and a low-resistivity quenched state, charge glass, is achieved experimentally via heat pulses supplied by optical or electrical means in organic conductors $\theta$-(BEDT-TTF)$_2$$X$. Switching that is one order of magnitude faster is observed in another isostructural material that requires faster cooling to kinetically avoid charge crystallization, indicating that the material's critical cooling rate can be useful guidelines for pursuing a faster correlated-electron PCM function.

Electron spin resonance study on charge ordering and spin ordering in nanocrystalline Bi0.2Ca0.8MnO3

Bi-based manganites Bi0.2Ca0.8MnO3 samples with different paiticle sizes were prepared by the sol-gel technique. The effect of paiticle size on the charge ordering (CO) and spin correlations of Bi0.2Ca0.8MnO3 was investigated by electron spin resonance (ESR). The variation in ESR intensity with temperature shows that the long-range CO transition is suppressed by the size reduction, and completely disappears as the paiticle size is reduced to 40 nm. In the paramagnetic (PM) region, the ESR intensity is fitted by the Arrhnius formula. The result shows that the activation energy is significantly enhanced with decreasing of paiticle size, especially in the 40 nm sample, indicating the enhancement of ferromagnetic (FM) correlations. However, the temperature dependence of ESR line width displays a typical CO characteristics for all samples. Thus, it is suggested that there is the short-range CO state in the 40 nm sample, while the long-range CO transition is completely suppressed. It is found that the onset temperatures of CO states are almost the same in all samples, indicating that the strength of CO correlations is not influenced by size reduction in this compound. A positive Curie-Weiss (CW) temperature is obtained from the line width in high-temperature PM regime, which confirms the existence of FM correlations in this system. Moreover, the value of CW temperature shows a significant decrease with particle size reduction, which indicates that the FM interactions can be weakened by size reduction in this system. Based on the research of ESR intensity and line width, it is concluded that the suppressed CO cannot be attributed to the enhancement of double-exchange FM interactions in Bi0.2Ca0.8MnO3 nanoparticles. To explain these behaviors, a core-shell model based on surface effect is proposed. In nanosized CO manganites, the disordered surface spins destroy the collinear antiferromagneitc (AFM) configuration, and favor FM surface layer coupled with the inner AFM core. With the reduction of paiticle size, the weakening of long-range AFM CO is more significant than that of short-range FM ordering due to the increase of surface spin disorders. With the reduction of paiticle size, the FM ordering will gradually dominate in the competition between FM ordering and AFM ordering, which results in the disappearance of CO transition peak in the ESR intensity curve.

Melting of the charge-ordered state under substantially lower magnetic field in structurally improved Pr1−Ca MnO3 (x=0.3−0.5) thin films

Abstract We report melting of the charge-ordered state in low bandwidth manganite PCMO ( x =0.3–0.5) thin films under substantially lower magnetic field. Two different in situ post-deposition annealing conditions were applied on the PCMO thin films grown by pulsed laser deposition at a deposition temperature of 500 °C, which is significantly lower compared to the traditional deposition temperature for complex oxides. Thorough structural and magnetic characterization of these two films show improved crystalline and magnetic properties in PCMO thin films annealed at a lower temperature that leads to stronger ferromagnetic interaction at the cost of charge- and orbital-ordered states. In these structurally improved film a larger irreversible metamagnetic transition can be observed and the onset magnetic field required to melt the charge-ordered state reduces to only around 2 T at 70 K temperature in the sample with x =0.4 and nearly around 2.5 T for x =0.5. Additionally, the in situ post-deposition annealing at 500 °C decreases the insulator–metal transition about 2.5 T when compared to the samples annealed at 700 °C. Therefore, a larger colossal magnetoresistance effect is observed under a moderate value of applied magnetic field in these PCMO films with better crystalline order. This result is of extreme importance for the future generation of oxide electronics using phase-change property of strongly correlated materials.

Epitaxially Textured Pr0.6Ca0.4MnO3 Thin Films Under Considerably Low Substrate Temperature

We report the growth of well-crystallized and epitaxially textured Pr0.6Ca0.4MnO3 thin films on SrTiO3 substrates by pulsed laser deposition at considerably low substrate temperatures, as low as 450 ̊C, without high-temperature post-annealing treatments. Although a strong ferromagnetic interaction as well as a large irreversible metamagnetic transition with a training effect have been observed for films grown at 450 ̊C, the in-plane and out-of-plane lattice ordering is slightly improved with increasing substrate temperature. Therefore, a lowest magnetic field of 2 T for melting the insulating charge-ordering state at 70K has been observed for films grown with the substrate temperature between 550 ̊C and 600 ̊C. The formation and growth of Pr0.6Ca0.4MnO3 on SrTiO3 substrate at exceptionally low substrate temperature is qualitatively modelled by the combination of the kinetic energies and redox potentials of the components of the ablation plasma, while the heat flow from the substrate is assumed to be less important.

Terahertz-infrared spectroscopy of overdoped manganites La1−xCaxMnO3

Terahertz and infrared spectra of dielectric permittivity and optical conductivity of overdoped manganites La1−xCaxMnO3 (0.5≤x≤1) have been measured at frequencies ν from 4cm−1 to 700cm−1 and at temperatures T from 5K to 300K. The samples were prepared in the form of bulk polycrystals (ceramics) and epitaxial films (free-standing and on MgO substrates). Strongly asymmetric absorption bands have been found in the compounds which are in charge-ordered state (0.5<x<0.85) and have commensurate calcium doping. The bands are located close to the position of lowest energy van Hove singularity of the folded Brillouin zone. The bands are associated with a boson peak, i.e., with the excess of lattice acoustical vibrational states over the standard Debye contribution (~ν2). In addition, Raman scattering measurements on ceramic samples have been conducted, which confirmed our assignment of the bands’ origin. We firmly identify contribution to the low energy spectra due to quasi-free charge carriers. In the paramagnetic phase all samples show a Drude-like response corresponding to the band type of charge transport. At low temperatures, for the range of dopings 0.5≤x<0.85, the conductivity spectra show signatures of variable-range hopping mechanisms of transport.

Hard X-ray photoemission study of the Fabre salts (TMTTF)2X (X = SbF6 and PF6)

Core-level photoemission spectra of the Fabre salts with X = SbF6 and PF6 were taken using hard X-rays from PETRA III, Hamburg. In these salts TMTTF layers show a significant stack dimerization with a charge transfer of 1e per dimer to the anion SbF6 or PF6. At room temperature and slightly below the core-level spectra exhibit single lines, characteristic for a well-screened metallic state. At reduced temperatures progressive charge localization sets in, followed by a 2nd order phase transition into a charge-ordered ground state. In both salts groups of new core-level signals occur, shifted towards lower kinetic energies. This is indicative of a reduced transverse-conductivity across the anion layers, visible as layer-dependent charge depletion for both samples. The surface potential was traced via shifts of core-level signals of an adsorbate. A well-defined potential could be established by a conducting cap layer of 5 nm aluminum which appears “transparent” due to the large probing depth of HAXPES (8–10 nm). At the transition into the charge-ordered phase the fluorine 1s line of (TMTTF)2SbF6 shifts by 2.8 eV to higher binding energy. This is a spectroscopic fingerprint of the loss of inversion symmetry accompanied by a cooperative shift of the SbF6 anions towards the more positively charged TMTTF donors. This shift does not occur for the X = PF6 compound, most likely due to smaller charge disproportion or due to the presence of charge disorder.

Kinetics of the charge ordering in magnetite below the Verwey temperature

In this work the kinetics of the charge ordering in magnetite (Fe3O4) below the Verwey transition temperature TV is investigated in time and energy domain. After excitation by a one-second voltage pulse to destruct the charge-ordered state below TV, an alternating current (AC) is used to perturb its recovery process. Upon warming up to above a temperature Tr(< TV) the charge order recovers despite the ac perturbation, because the temperature-dependent relaxation time becomes shorter than the polarity change of the ac. From the frequency dependence of Tr(f), an activation energy of ΔE = 126 meV is extracted. Below Tr the real part of the ac conductance Greal follows the relation Greal ∼ fα with α = 0.98 ± 0.18, suggesting that the charge reordering is driven by correlated hopping transport. The relaxation time τ = 1/f(Tr) of the charge-ordered state is determined for temperatures between 70 and 98 K and is extrapolated to τ(TV) = 1.6 ms, continuously slowing down its dynamics as the temperature is decreased.

A real space description of magnetic field induced melting in the charge ordered manganites: II. The disordered case

We study the effect of A site disorder on magnetic field induced melting of charge order (CO) in half doped manganites using a Monte-Carlo technique. Strong A-site disorder destroys CO even without an applied field. At moderate disorder, the zero field CO state survives but has several intriguing features in its field response. Our spatially resolved results track the broadening of the field melting transition due to disorder and explain the unusual dependence of the melting scales on bandwidth and disorder. In combination with our companion paper on field melting of charge order in clean systems we provide an unified understanding of CO melting across all half doped manganites.

Transitions between photoinduced macroscopic quantum states in 1T-TaS2 controlled by substrate strain

Controllable switching to and from metastable states of matter using electromagnetic fields could potentially revolutionize electronic logic and memory devices. Here, we investigate the effect of two-dimensional strain on switching between a photoinduced “hidden” state and a stable charge-ordered state in ultrathin 1T-TaS2 crystal films on different substrates, photoexcited by 35 fs laser pulses. The differential contraction of the sample and the substrate shows a very large and negative strain coefficient on the hidden state transition, implying that the stability of the hidden state could be markedly increased by tensile strain. Other transitions are not strongly affected by tensile strain.

Terahertz–infrared electrodynamics of overdoped manganites La1–xCaxMnO3

We have measured low-temperature terahertz and infrared spectra of La1–xCa xMnO3 with x = 0.5; 0.6; 2/3, 0.7; 3/4, 0.85; 0.9; 0.95; 0.98 and x = 1 in the form of ceramics and epitaxial films. In the charge-ordered state for commensurate dopings (x = 2/3 and 3/4), we observed an absorption band at frequencies corresponding to the position of the lowest energy van Hove singularity in the folded Brillouin zone. The band is assigned to the boson peak. We observed qualitatively the same boson peak in Raman spectra of La1/3Ca2/3MnO3 ceramics. In antiferromagnetic phase, for the 0.5 ≤ x < 0.85 range of doping, the conductivity mechanism gradually changes from a hopping-like transport at low temperatures to the Drude behavior at elevated temperatures in the paramagnetic phase. For 0.85 ≤ x < 1 (canted antiferromagnetic phase), the electrons behave as Drude-like carriers in the whole temperature interval 5–300 K.

C13NMR study of the charge-ordered state near the superconducting transition in the organic superconductorβ′′−(BEDT-TTF)4[(H3O)Ga(C2O4)3]·C6H5NO2

The charge-ordered state in a quasi-two-dimensional organic superconductor ${\ensuremath{\beta}}^{\ensuremath{'}\ensuremath{'}}\text{\ensuremath{-}}(\text{BEDT-TTF}){}_{4}[({\mathrm{H}}_{3}\mathrm{O})\text{Ga}({\mathrm{C}}_{2}{\mathrm{O}}_{4}){}_{3}]\ifmmode\cdot\else\textperiodcentered\fi{}{\mathrm{C}}_{6}{\mathrm{H}}_{5}{\mathrm{NO}}_{2}$ has been investigated using $^{13}\mathrm{C}$ NMR spectroscopy in a high magnetic field. Enhanced resolution of the high-field spectrum allows us to estimate the site charges in the low-temperature charge-ordered state below $8.5$ K. The observed strong increase in the charge imbalance suggests that superconductivity appears on the electronic state with a strong charge modulation. From the NMR intensity ratio of signals from the charge-rich and charge-poor sites, we discuss a possible charge pattern with threefold modulation, which could be stabilized by the effects of electron-electron correlations.

Double charge ordering states and spin ordering state observed in a RFe2O4 system.

Charge, spin, and lattice degrees of orderings are of great interest in the layered quantum material RFe2O4 (R = Y, Er, Yb, Tm, and Lu) system. Recently many unique properties have been found using various experimental methods. However so far the nature of the two-dimensional (2D) charge ordering (CO) state is not clear and no observation of its fine structure in energy has been reported. Here we report unambiguous observation of double 2D CO states at relatively high temperature in a polycrystalline Er0.1Yb0.9Fe2O4 using Raman scattering. The energy gaps between the 3D and the double 2D states are 170 meV (41.2 THz) and 193 meV (46.6 THz), respectively. We also observed a spin ordering (SO) state at below 210 K with characteristic energy of 45 meV (10.7 THz). Our investigation experimentally identified new fine structures of quantum orders in the system, which also extends the capability of optical methods in investigating other layered quantum materials.

Effect of silver doping on the surface of La5/8Ca3/8MnO3 epitaxial films

Thin film manganese oxides (manganites) display remarkable properties, such as colossal magnetoresistance and charge ordered phases, and became a focal point of research in the past two decades owing to potential applications ranging from oxide spintronics to resistive switching-based memories. LaxCa1−xMnO3 (LCMO), a widely studied manganite, is known to substantially improve its transport properties when doped with Ag. However, despite the abundance of studies on LCMO, the effect of silver on the surface structure is unknown. Here, through in-situ methods, scanning tunneling microscopy (STM) is performed on La5/8Ca3/8MnO3 films grown by pulsed laser deposition. Films doped by silver, as confirmed by in-situ X-ray photoelectron spectroscopy, display large-scale reconstructions, interpreted as being of type (√10 × √10)R18.4°, while films lacking silver display a (√2 × √2)R45° reconstruction that may be associated with a surface charge-ordered state. It is posited that the possible cause of the varied reconstructions is due to a vacancy ordering on top of the existing (√2 × √2)R45° reconstruction. These studies highlight the influence of Ag on the surface structure, and therefore a route towards modifying the surface properties of manganites.

Temperature and polarization dependence of Fe L3-edge X-ray absorption spectra of LuFe2O4

The charge ordering of Fe2+ and Fe3+ ions in LuFe2O4 induces ferroelectricity below 330K. In this study, the X-ray absorption spectra (XAS) at the Fe L2,3-edge are investigated to study charge ordered- and disordered-states. The polarization dependence of XAS revealed that the energy level of (dxy, dx2–y2) is lower than that of (dyz, dzx). In the charge-ordered state, the Fe L3-XAS shows a double-peak structure, which is a characteristic of Fe2+ and Fe3+ ions. In the high-temperature region, the double-peak structure becomes a broad single peak, because of overlap with the thermally excited state.

Vortices and charge order in high-Tcsuperconductors

We theoretically investigate the vortex state of the cuprate high-temperature superconductors in the presence of magnetic fields. Assuming the recently derived nonlinear $\sigma$-model for fluctuations in the pseudogap phase, we find that the vortex cores consist of two crossed regions of elliptic shape, in which a static charge order emerges. Charge density wave order manifests itself as satellites to the ordinary Bragg peaks directed along the axes of the reciprocal copper lattice. Quadrupole density wave (bond order) satellites, if seen, are predicted to be along the diagonals. The intensity of the satellites should grow linearly with the magnetic field, in agreement with the result of recent experiments.